The present invention relates to electrode materials for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries using the materials.
Lithium secondary batteries (hereinafter referred to as xe2x80x9cbatteriesxe2x80x9d) use, for their negative electrodes, metal lithium or elements and compounds that have electric potentials similar to that of lithium and reversibly intercalate and de-intercalate lithium ions. For the positive electrodes, compounds that reversibly intercalate and de-intercalate lithium or lithium ions and have electric potentials higher than that of lithium are used. These compounds are used as electrode material for electromotive reaction, or xe2x80x9cactive materialsxe2x80x9d. As non-aqueous electrolytes such as organic or solid electrolytes are used as their electrolyte, such batteries are called non-aqueous electrolyte secondary batteries.
Featuring large electromotive force and high energy density, in recent years, the above batteries have found a wide range of applications not only in such small equipment as small electric appliances, mobile communications equipment, mobile electronic equipment, personal digital assistants, but also in small domestic power storage devices and power sources for motor bicycles, electric vehicles, hybrid cars, and the like. For these batteries, various forms such as a coin, cylinder, square, and sheet are employed according to their uses.
While batteries using metal lithium for their negative electrodes have an advantage of high energy density, dendrite deposits on the surfaces of the negative electrodes during charging. And the deposition of the dendrite is known to cause many of the following problems. For example, when dendrite penetrates a separator, an internal short circuit or shorter battery life is caused. On the surface of dendrite having a large specific surface area and high reactivity, a surface layer like a high resistance solid electrolyte is produced by the reaction of the solvent in the electrolytic solution. The layer increases the internal resistance. Moreover, the production of the layer involves an increase in the number of particles apart from electronic conduction network in the electrode plates, decreasing charge/discharge efficiency.
In order to address the above problems of a shorter battery life and lower reliability, the technologies using carbon materials of various forms or such compounds as a spinel-type lithium-containing transition metal that can intercalate and de-intercalate lithium ions without producing dendrite are developed and put into use.
Meanwhile, as the materials for positive electrodes, many kinds of lithium-containing compounds that reversibly intercalate and de-intercalate lithium ions at an electric potential higher than that of lithium are known. Among such materials, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), spinel-type lithium manganate (LiMn2O4) (hereinafter referred to as xe2x80x9cmanganese spinelxe2x80x9d) exhibit an electrode potential ca 4V higher than that of lithium, and all of them can constitute batteries with high energy density. The theoretical capacity of lithium cobaltate and lithium nickelate is 280 mAh/g, and that of manganese spinel is 148 mAh/g. The capacity of manganese spinel is smaller than that of lithium cobaltate and lithium nickelate. However, manganese spinel creates less heat stability hazards than lithium nickelate, and other materials, and manganate is an abundant and cheap natural resource. Thus, the manganese spinel is expected as a highly practical electrode material.
However, it is well known that when a battery using manganese spinel as its electrode material is stored, or repeatedly charged and discharged at high temperatures, manganese elutes from active substance and causes a phenomenon of considerably deteriorating its capacity and characteristics after high-temperature storage and cyclic charge/discharge tests.
In order to improve unstable states of the manganese spinel at high temperatures, some techniques of stabilizing the manganese spinel by substituting a part of its octahedral sites occupied by the manganese in the crystal structure of the manganese spinel with other elements have been developed.
Techniques that have been employed include:
substituting a part of the octahedral sites occupied by the manganese with transition metal elements such as cobalt and chromium; partially substituting the manganese sites for lithium that has added more than determined by its stoichiometric composition ratio, during the synthesis of active materials; and
substituting a part of the manganese sites with typical elements such as aluminum and magnesium.
The aforementioned techniques called xe2x80x9csolid state substitutionxe2x80x9d have provided slight improvement in high-temperature storage and cyclic charge/discharge characteristics. The high-temperature storage characteristic, and the like, have been improved probably because substituting a part of manganese occupying the octahedral sites with substitution elements stabilizes the crystal structure having common oxygen configurations, thus inhibiting elusion of the manganese. However, at present, the electrode materials are not yet reached to a fully practical level.
The present invention clarifies specific structural requirements for stabilizing lithium-containing transition metal oxides, and moreover, intends to improve high-temperature storage and cyclic charge/discharge characteristics of the batteries by using improved spinel compounds for their electrodes.
As a result of detailed research of the inventor of the present invention, the reason why even the aforementioned methods of substituting a part of manganese sites cannot obtain fully practical characteristics has been thought as follows:
The partially substituting elements uniformly occupy the octahedral sites of the spinel structure, and each tetrahedral site adjacent to the octahedral sites is occupied mainly by lithium. Thus, the substituting elements that should contribute to the stabilization of the crystal structure cannot exist in octahedral and tetrahedral sites, structural units of the spinel structure, at the same time.
Therefore, the present invention discloses other electrode materials and the batteries using such materials. The disclosed materials are characterized in that a lithium-containing transition metal oxide having a spinel crystal structure includes, in a part of its crystal structure, different spinel composition consisting of the configurations of different elements in combination that can constitute a spinel structure using common oxygen configurations.
In other words, the present invention intends to stabilize electrode materials by providing a stable spinel structure other than the spinel structure of the lithium-containing transition metal oxide using common oxygen configurations so that the lithium-containing metal oxide includes, in a part thereof, the stable spinel structure.
The conventional technique intended to stabilize the spinel crystal structure using its common oxygen configurations by uniformly substituting a part of manganese in the octahedral sites with substitution elements.
On the other hand, the present invention is characterized by the fact that providing at least two kinds of typical elements in a lithium-containing transition metal oxide at the same time in a ratio that can form a spinel structure allows the formation of a spinel structure of the foregoing typical elements in the lithium-containing transition metal oxide using the oxygen configurations common to them. The present invention allows a lithium-containing transition metal oxide to be stabilized by providing a stable spinel structure portion in the lithium-containing transition metal oxide.
The composition of the lithium-containing transition metal oxides used in the present invention has no limitation except that they have spinel structures. For example, the effect of stability can be expected to LiMn2O4 having a molar ratio of Li:Mn=1:2, Li1.2Mn1.8O4 having a molar ratio of Li:Mn=1.2:1.8, and LiMn1.6Ni0.4O4 having a molar ratio of Li:Mn:Ni=1:1.6:0.4.
The spinel compounds of the present invention can be synthesized at the step of synthesizing spinel materials by adding, to a material of a lithium-containing transition metal oxide, a plurality of materials containing typical elements for forming spinel structures in combination at predetermined mixing ratios, and firing the obtained mixtures.